1. Field of the Invention
The present invention relates to an active energy ray-curable coating composition for forming a film having excellent hardness, abrasion resistance, transparency and antistatic properties by irradiating with active energy rays.
2. Discussion of the Background
Plastic products such as polycarbonate, polymethylmethacrylate, polyethylene terephthalate, polybutylene terephthalate, styrene type resins including acrylonitrile.butadiene.styrene (ABS) resin, methylmethacrylate.styrene (MS) resin and acrylonitrile.styrene (AS) resin, vinyl chloride type resins and cellulose acetate including triacetyl cellulose, and their resin substrates, are excellent in their lightness, easy processability, impact resistance and the like, and are accordingly applicable to various uses including containers, instrument panels, wrapping materials, various housing materials, optical disk substrates, plastic lens, liquid crystal displays, plasma displays and other display substrates.
However, these plastic products have poor surface hardness and are easily damaged. Transparent resins such as polycarbonate and polyethylene terephthalate have defects of losing inherent satisfactory transparency or external appearance, and they are hardly usable in such a field as to require an abrasion resistance.
Therefore, an active energy ray-curable hard coating material (covering material) imparting a satisfactory abrasion resistance to the surfaces of these plastic products is demanded. However, cured layers of commercially available active energy ray-curable hard coating materials have a high surface resistivity and have a large defect of generating static electricity. The generation of static electricity accelerates attachment of dust and dirt, and loses beautiful appearance or transparency of a product.
In order to solve these problems, a method for coating an active energy ray-curable coating material or a method for coating an antifouling coating material, which provides film imparting abrasion resistance and antistatic properties to the surface of a plastic product, have been variously studied. In addition, it has been considered to provide a plastic product having an abrasion-resistant antistatic layer on the surface of a plastic product and having a transparent film having an adhesive layer attached to the back side.
However, these methods have been still unsatisfactory in abrasion resistance.
Accordingly, various active energy ray-curable coating agents providing more satisfactory hardness and abrasion resistance than conventional methods have been recently proposed. For example, JP-A-11-309814 discloses coating at least two layers of coating agents, in which an inorganic film-formable coating agent such as polysilazane is used as the outermost layer, thereby greatly improving abrasion resistance. However, there are problems that it is difficult to make a thick film with the inorganic coating agent and that productivity becomes poor because of coating at least two layers.
On the other hand, by coating at least two layers of coating agents having different elasticities, it has been tried to improve hardness and abrasion resistance. For example, JP-A-2000-52472 discloses providing a second layer of a coating agent having an elasticity higher than an elasticity of a coating agent of a first layer to obtain a coating film having a high hardness. On the other hand, JP-A-2000-214791 discloses providing a second layer of a coating agent having an elasticity lower than an elasticity of a coating agent of a first layer to obtain a coating film having a high hardness. In these manners, an apparent hardness seems to be produced by absorbing an impact and avoiding a concentration of stress, but when a total thickness of a coating film exceeds 10 microns, at least two layers are coated, thus raising a problem such as a poor productivity.
Also, JP-A-2000-219845 discloses coating a methacrylic polymer as a first layer and then laminating a cured coating layer comprising a colloidal silica and an organosiloxane resin of a hydrolysis condensate of a specific silicate as a second layer, thereby producing a coating film having a total thickness of at most 10 microns and having an excellent abrasion resistance, but a problem of a poor productivity is still caused since at least two layers are coated.
Further, JP-A-2000-15734 discloses to improve hardness by using a coating agent having a low elasticity component such as a urethane acrylate oligomer blended with a polyfunctional acrylate and then imparting antistatic properties by providing a multi-layer coating of an electroconductive layer such as an ITO (indium.tin composite oxide) layer and an inorganic layer such as an SiO2 layer. However, when such an antistatic layer is provided, a multi-layer coating must be provided, thus raising a productivity problem.
On the other hand, various coating agents for achieving excellent hardness and abrasion resistance even by monolayer coating have been studied. Accordingly, a composition comprising colloidal silica and polyfunctional acrylates, a hydrolysate.condensate composition comprising colloidal silica and a specific silicate, a curable resin composition comprising these materials and a polyfunctional acrylate, an epoxy resin, a phenoxy resin or the like, and a composition comprising these materials and an acrylic resin polymer have been heretofore widely studied as an organic.inorganic composite coating agent, but all of these compositions have provided problems such as not achieving satisfactory hardness and abrasion resistance, poor stability of a coating solution, and poor environmental durabilities (moisture resistance, heat resistance or the like) of a cured film.
As compared with these prior arts, an active energy ray-curable coating agent comprising a compound obtained by reacting a polyfunctional acrylate with a colloidal silica as a base as disclosed in JP-A-5-287215 and JP-A-9-100111 provides more satisfactory hardness and abrasion resistance even by monolayer coating as compared with a conventional organic inorganic composite coating agent, but antistatic properties are still low and dusts or soils are easily attached.
As a coating agent for providing antistatic properties, JP-A-10-279833 and JP-A-2000-80169 disclose a technique of blending an organic cationic material such as a quaternary ammonium salt structure, and JP-A-2000-95970 discloses a technique of blending an anionic material such as an organic sulfonate. In addition, a composite of an inorganic ion-conductive material and a nonionic material such as a polyalkylene glycol, a technique of using a silicate, a technique of dispersing an inorganic electrocondutive filler, a technique of blending an organic π electron conjugate polymer and the like have been studied.
However, in these inventions, hardness and abrasion resistance are not satisfactory. In systems containing colloidal silica effective for realizing a high hardness, some techniques have been proposed to realize hardness and antistatic properties, but all of them include a technique of adding an electroconductive filler (such as metal fine particles or electroconductive composite oxides) as an antistatic property-imparting component, and there are many restrictions to applicable uses because of problems of coloring and hardly making a thick film.
On the other hand, by combining an organic cationic material and a colloidal silica, it is possible to make a thick film, but due to an ionic interaction between anions on the surface of the colloidal silica and the organic cationic material, a dispersion state becomes easily unstable, and it has been difficult to achieve homogeneous blending. As a method for improving this problem, it has been proposed to combine an organoalkoxysilane and a polyfunctional acrylate as described in JP-A-5-179160 and JP-A-5-179161, and it has been disclosed that it is possible to homogeneously mix colloidal silica and an antistatic property-imparting agent such as an organic cationic material. The cured material derived therefrom is excellent in hardness and antistatic properties. However, its surface resistivity is on the order of 1010 (Ω) and antistatic properties are still unsatisfactory, and abrasion resistance is also still unsatisfactory. Particularly, with regard to an organic cationic material having a high performance of realizing antistatic properties in the order of 108 to 109 (Ω) presently demanded in many uses, it has been difficult to homogeneously mix and it has been quite difficult to freely blend.
On the other hand, it is relatively easy to homogeneously mix a colloidal silica with other antistatic agents (such as an organic anionic material, an organic nonionic antistatic agent or a silicate), but a compatibility with an organic high hardness-imparting component such as a polyfunctional acrylate is poor and a transparency is lost (organic anionic material), and an antistatic property level achieved is unsatisfactorily on the order of 1010 to 1012 (Ω) (an organic nonionic antistatic agent or a silicate) and they are not satisfactory for practical use. The present inventors heretofore proposed an improved method of introducing an organic anionic group into a polyfunctional acrylate itself in the former case (for example, as described in JP-A-7-41695) to improve transparency, but its performances were still unsatisfactory in view of presently required levels of hardness and antistatic properties. Also, the present inventors proposed to improve performances by combining a silicate and a polyfunctional acrylate having a carboxyl group in the latter case (for example, as described in JP-A-8-325474), but its performances were still unsatisfactory in view of presently required levels of hardness and antistatic properties.